Commercial and business jet aircrafts have engine inlets tailored for specific aerodynamic and acoustic design considerations. For example, some aircraft engine nacelle inlets or intake cowlings may include acoustic liners mounted therein to meet noise regulatory requirements. Typical inlet acoustic panels are designed with uniform impedance from an inlet forward bulkhead to an inlet aft bulkhead. This configuration does not always adequately absorb sound waves propagating within the inlet. As sound is attenuated in the inlet from a fan face forward toward the inlet, the uniform impedance is no longer optimum along the entire length of the inlet acoustic panel.
Therefore, there is a need for an improved method and apparatus for attenuating engine noise that overcomes the limitations of the prior art.